The present invention relates generally to ways to convert unreacted hydrogen present in a fuel cell coolant into water, and more particularly to ways to reduce the buildup of pressure within a coolant tank so that oxygen used to react with the hydrogen can be introduced into a reaction chamber for the catalytic removal of the hydrogen regardless of the fuel cell power output.
In many fuel cell systems, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. An appropriate catalyst (for example, platinum) ionizes the hydrogen into a proton and electron on the anode side such that upon subsequent combination of the proton with oxygen and the electrons at the cathode side, electric current is produced with high temperature water vapor as a reaction byproduct.
In one form of fuel cell, called the proton exchange membrane (PEM) fuel cell, an electrolyte in the form of a membrane is sandwiched between two electrode plates that make up the anode and cathode. The membrane may be formed from a perfluorinated polymer containing sulphonic acid, which allows the formation of negatively charged transfer sites that can conduct positively charged ions formed in the anode. In the case of hydrogen fuel as one of the reactants, the positively charged hydrogen ions pass through the membrane to react with oxygen and electrons present on the cathode. This layered structure of membrane sandwiched between two electrode plates is commonly referred to as a membrane electrode assembly (MEA), and forms a single fuel cell. Many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. While the present invention is especially applicable to the PEM fuel cell, it will be appreciated by those skilled in the art that the use of other fuel cell configurations with the present invention is also within the purview of the present disclosure.
The reaction at a fuel cell cathode is exothermic, giving off a significant amount of heat. Accordingly, fuel cells with larger power outputs, such as in automotive applications, where tens (if not hundreds) of kilowatts may be required, are equipped with supplemental (usually liquid-based) cooling systems to prevent damage to the fuel cell components. The coolant can be supplied through various sources, such as a dedicated circuit including a cooling tank. Coolant make-up components may be further included to augment the supply of coolant lost due to leakage, evaporation or the like. In one form of coolant delivery, coolant distribution plates with flow channels can be placed between the various anode and cathode plates to conduct away heat produced by the aforementioned reaction. In addition to cooling, humidification schemes are often necessary to maintain a proper water balance in the fuel cell. One use for such humidification is to keep the membrane from drying out and producing a concomitant reduction in proton conductivity.
The high pressures and closely-spaced relationship between both the reactant and coolant flowpaths means that some fluid crosstalk and related contamination will occur. Thus, excess (unreacted) quantities of hydrogen may build up in the coolant. It is desirable to reduce the presence of this unreacted hydrogen in the coolant. One way to achieve this reduction is to pass the hydrogen-contaminated coolant through a catalytic reaction between the hydrogen and an oxygen-bearing fluid (such as ambient air from an air ventilation flow) to convert the hydrogen into water in a manner similar to that occurring on the fuel cell cathode. Unfortunately, this approach is disadvantageous because the presence of the relatively dry ventilation air that is necessary to sustain the reaction at the catalyst tends to draw away the much-needed humidity (and water) from the cooling system, necessitating coolant make-up. In addition, the buildup of water resulting from the catalytic conversion of the excess hydrogen is problematic because it increases the pressure within a closed cooling system, which tends to hamper further operation of the hydrogen-removing reaction. Furthermore, if the pressure within the cooling system were allowed to consistently remain above that of the cathode or anode loop of the fuel cell, small leakages of cooling fluid (which may contain glycol or related materials) from the coolant system would find their way into the cathode or anode loop, with a potential loss in fuel cell durability. In addition, it is important for service to have access to the open coolant tank. Yet the direct contact of service personnel to hydrogen is undesirable in an open system, as an ignition source could lead to a fire.
Accordingly, there exists a need to ensure thorough removal of unreacted hydrogen from the cooling system of an operating fuel cell. There further exists a need to relieve the pressure buildup in a cooling system so that an oxygen-bearing fluid can be delivered to the catalytic reaction site regardless of the power setting in the fuel cell.